After just (finally) watching Fury (took me this long to see it?!) I thought of concealed units during the scene with hidden AT guns. I'm still new to TOI so bear with me, it seems that concealed units are not truly hidden since they are obvious on the board (just untargetable unless spotted via some action). Without numbered hexes there is no way to conceal units from your opponent but I thought of using the numbered transport tokens:

This only works with two sets of tokens, one set on the board and one in the player's off-board area. The opponent will not know which token is the hidden unit(s) but they will know there are some there. I figure the concealed units with AT guns would only have a three hex range (three also for lone squads against other squads) and can choose when to fire (op mode) or make a concentrated attack. Of course they would be revealed, and also unable to move once placed as concealed (unlike normal concealed units that can move and remain concealed as long as there is cover terrain or out of line of sight of enemy units) and maybe unable to move the AT gun unless the squad abandons it. Opponents could also have the choice of attempting suppressive fire on hexes with tokens to reveal units (which means the tanks could reveal the hidden units outside the range of those units).

Just some ideas, I'm sure others have discussed this so throw me a word or two if you like.

I'm not sure what you mean. My example shows one set of tokens 5-8 on the board but turned down so the opponent does not know which number is where. The corresponding off-board tokens (another set 5-8) have units on two of the numbers. The off-board numbers under the units are visible so the opponent can see there is no cheating once the on board tokens are placed. Of course the player hiding the units would have to remember which numbers are where on the board. The question tokens have no numbers so it would be hard to tell where the hidden units are on the board. This method also avoids placing something in the player's area that covers the off-board units and taking up space (although a cover could hide the type of unit if played that way). This type of rule would mean hidden units are placed and cannot move unlike normal concealed units would.

It's just an interesting variation to me. In Fury the Americans knew there were AT guns in the tree-line but not where (or exact type). The Germans attacked with one AT gun but waited with the other. TOI rules like other games you can see exactly where hidden units are. I've played Art of Tactic which uses numbered hexes that make it easier to hide units on the board.

Yes, fog of war, very difficult to introduce good rules for.Concealed units work as good as we could hope for. This method of yours sound very good indeed. It should also be mentioned that the 'sewer' movement in Stalingrad expansion is very good and adds a lot of fog of war without much trouble.

I dream of setting up a double blind scenario at one point, but it takes effort.

Been using this method for years! Never thought of posting for ToI. For me, it came from miniature gaming and using counters to represent units or lack there of for concealment and not allowing my opponent to see the real figure on the board. It was a natural transfer over to ToI.